JUPITER'S Moons - EnchantedLearning.com Jupiter's Moons Activities, Web Links JUPITER'S MOONS Jupiter has four large moons and dozens of smaller ones (there are about 67 known moons so far). Galileo first discovered the four largest moons of Jupiter, Io, Europa, Ganymede, and Callisto in 1610, using a 20-power telescope; these moons are known as the Galilean moons. The moons of Jupiter are (in order by their distance from Jupiter): Metis, Adrastea, Amalthea, Thebe, Io, Europa, Ganymede (the biggest), Callisto (the second biggest), Leda (the smallest), Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, Sinope, and many newly-discovered moons that haven't been named yet. METIS Metis is the closest moon to Jupiter. Metis is 25 miles (40 km) in diameter and orbits 79,500 miles (128,000 km) from Jupiter, within its main ring.

Metis and the next moon, Adrastea, are probably the source of the dust in this ring. Metis has a mass of 9 x 1016kg. It orbits Jupiter in 0.294780 (Earth) days; this is faster than Jupiter rotates on its axis. Metis was discovered by Stephen Synnott (Voyager 2) in 1980. ADRASTEA Adrastea is the second-closest moon to Jupiter. Adrastea is 12 miles (20 km) in diameter and orbits 80,000 miles (129,000 km) from Jupiter, within its main ring. Adrastea and the first moon, Metis, are probably the source of the dust in this ring. Adrastea has a mass of 1.91 x 1016kg. It orbits Jupiter in 0.29826 (Earth) days; this is faster than Jupiter rotates on its axis. Adrastea was discovered by D. Jewitt and E. Danielson (Voyager 2) in 1979.

AMALTHEA Amalthea is the third-closest moon to Jupiter and the reddest object in our solar system . Amalthea is 145 x 91 x 83 miles (232 x146 x134 km) in diameter and orbits 112,700 miles (181,300 km) from Jupiter, within the faint Gossamer ring. Amalthea and Thebe likely provide the dust for the Gossamer ring. Amalthea has a mass of 7.2 x 1021kg. It orbits Jupiter in 0.49817905 (Earth) days and is in synchronous rotation (always keeping the same side facing Jupiter). Amalthea gives off more heat than it receives from the Sun. Amalthea was discovered by Edward Emerson Barnard in 1892. THEBE Thebe is the fourth-closest moon to Jupiter. Thebe is 68 x 56 miles (110 x 90 km) in diameter and orbits 138,000 miles (222,000 km) from Jupiter.

Amalthea and Thebe likely provide the dust for the Gossamer ring. Thebe has a mass of 8 x 1017kg. It orbits Jupiter in 0.6745 day (Earth) days and is in synchronous rotation (always keeping the same side facing Jupiter). Thebe was discovered by Stephen Synnott (Voyager 1) in 1980. THE GALILEAN MOONS Two sulfurous eruptions on Io. Pillan Patera, a volcanic caldera at the left, spews sulfur 86 miles above the surface. IO Io is a large, rocky, volcanically active moon of Jupiter. Its volcanoes spew out molten sulfur, making Io a very colorful moon. It is the innermost of Jupiter's four large moons and the third largest. It has a diameter of 1,942 miles (3,636 km), very close in size to our moon . Io's mean distance from Jupiter is 220,000 miles (422,000 km). It has a mass of 8.93x1022 kg.

It takes Io 1.77 days to orbit Jupiter. There is a doughnut-shaped plasma cloud around Jupiter near Io's orbit (known as the "Io plasma torus") This torus is caused by Jupiter's strong magnetic field, which strips ions from Io as it rotates; Io acts like an electrical generator. Io was discovered by Galileo and Marius (independently) in 1610. Ole Romer (1644-1710) was a Danish astronomer who, in 1675-1676, was the first person to demonstrate that the speed of light is finite. Romer did this by observing eclipses of Jupiter's moon Io as Jupiter's distance from Earth varied through the year. He noticed that the observed period of Io's orbit differed by about 20 minutes; he concluded that this difference was due to the extra distance that the light had to travel to Earth. His calculations put the speed of light at about 225,000 kilometers per second (it is really a bit faster, at 299,792 kilometers per second). EUROPA Europa is a large, dense, icy moon of Jupiter.

Europa is the smoothest object in our Solar System . Its surface is covered with long, crisscrossing trackways (but few craters) on water ice. Frozen sulfuric acid has been found on its surface. Europa's diameter is less than 2,000 miles (3,138 km), smaller than the Earth's moon . It takes Europa 3.55 days to orbit Jupiter (in a synchronous orbit). Its mean distance from Jupiter is about 420,000 miles (670,900 km). Its mass is 4.80x1022 kg. It was discovered by Galileo and S. Marius (independently) in 1610. GANYMEDE Ganymede is the largest moon of Jupiter, a large, icy, outer moon that is scarred with impact craters and many parallel faults. It has a diameter of about 3,400 miles (5,268 km) and orbits Jupiter at a mean distance of 664,000 miles (1,070,000 km). It has a magnetic field and probably has a molten iron core.

It takes Ganymede 171.75 hours (7.15 Earth days) to orbit Jupiter. Its mass is 1.48x1023 kg. It was discovered by Galileo and S. Marius (independently) in 1610. Ganymede is the largest moon in the solar system; it is also larger than the planets Mercury and Pluto. CALLISTO Callisto is a large, icy, dark-colored, low-density outer moon of Jupiter that is scarred with impact craters and ejecta. It has a diameter of about 3,000 miles (4800 km), the second-largest moon of Jupiter; it is roughly the size of Mercury . Callisto has the largest-known impact crater in the Solar System, Valhalla, which has a bright patch 600 km across and rings that go out to almost 3000 km. Callisto orbits Jupiter at a mean distance of 1,170,000 miles (1,883,000 km). Its mass is 1.08x1023 kg.

It takes Callisto 400.8 hours = 16.7 days to orbit Jupiter (in a synchronous orbit). Callisto was discovered by Galileo and S. Marius (independently) in 1610. LEDA Leda is Jupiter's ninth and smallest moon. Leda is 9.9 miles (16 km km) in diameter and orbits at an average of 6,900,000 miles (11,094,000 km) from Jupiter. Leda has a mass of 5.68 x 1015kg. It orbits Jupiter in 238.72 (Earth) days. Very little is known about Leda. Leda was discovered by Charles Kowal in 1974. HIMALIA Himalia is Jupiter's tenth moon. Himalia is 110 miles (170 km) in diameter and orbits 7,000,000 miles (11,480,000 km) from Jupiter. Himalia has a mass of 9.5 x 1018kg.

It orbits Jupiter in 250.5662 (Earth) days. Very little is known about Himalia. Himalia was discovered by C. Perrine in 1904. LYSITHEA Lysithea is Jupiter's eleventh moon. Lysithea is 15 miles (24 km) in diameter and orbits 7,200,000 miles (11,720,000 km) from Jupiter. Lysithea has a mass of 8 x 1016kg. It orbits Jupiter in 259.22 (Earth) days. Very little is known about Lysithea. Lysithea was discovered by S. Nicholson in 1938. ELARA Elara is Jupiter's twelfth moon. Elara is 50 miles (80 km) in diameter and orbits 7,250,000 miles (11,737,000 km) from Jupiter. Elara has a mass of 8 x 1017kg. It orbits Jupiter in 259.6528 (Earth) days.

Very little is known about Elara. Elara was discovered by C. Perrine in 1905. ANANKE Ananke is Jupiter's thirteenth moon. Ananke is 12.5 miles (20 km) in diameter and orbits 13,100,000 miles (21,200,000 km) from Jupiter. Ananke has a mass of 4 x 1016kg. It orbits Jupiter in 631 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Ananke. Ananke was discovered by S. Nicholson in 1951. CARME Carme is Jupiter's fourteenth moon. Carme is 18.5 miles (30 km) in diameter and orbits 13,800,000 miles (22,600,000 km) from Jupiter. Carme has a mass of 9 x 1016kg. It orbits Jupiter in 692 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter).

Very little is known about Carme. Carme was discovered by S. Nicholson in 1938. PASIPHAE Pasiphae is Jupiter's fifteenth moon. Pasiphae is 22 miles (36 km) in diameter and orbits 14,600,000 miles (23,500,000 km) from Jupiter. Pasiphae has a mass of 2 x 1023kg. It orbits Jupiter in 735 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Pasiphae. Pasiphae was discovered by P. Melotte in 1908. SINOPE Sinope is Jupiter's sixteenth moon. Sinope is 17.5 miles (28 km) in diameter and orbits 14,700,000 miles (23,700,000 km) from Jupiter. Sinope has a mass of 8 x 1016kg.

It orbits Jupiter in 758 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Sinope. Sinope was discovered by S. Nicholson in 1914. S/1999 J 1 (a provisional name) The seventeenth and outermost moon; S/1999 J 1 is the smallest-known moon orbiting a major planet. This moon is 3 miles (5 km) in diameter and has an irregular orbit roughly 15 million miles (24 million km) from Jupiter. It orbits Jupiter in 774 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). It was discovered by Robert S. McMillan et al (at the Spacewatch program at the University of Arizona ) in 2000.

Jupiter's Moons: Facts About the Largest Jovian Moons Jupiter's Moons: Facts About the Largest Jovian Moons By Kim Ann Zimmermann | July 5, 2012 05:33pm ET MORE This montage shows the best views of Jupiter's four large and diverse "Galilean" satellites as seen by the Long Range Reconnaissance Imager (LORRI) on the New Horizons spacecraft during its flyby of Jupiter in late February 2007. The four moons are, from left to right: Io, Europa, Ganymede and Callisto. The images have been scaled to represent the true relative sizes of the four moons and are arranged in their order from Jupiter. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute With a total of 67 known moons — including four large moons known as the Galilean satellites — Jupiter almost qualifies as a solar system unto itself. Most of the moons of Jupiter are small, with about 50 of the satellites being less than 6.2 miles in diameter.

The number of moons changes fairly frequently, with 66 and 67 being confirmed at the end of 2011. Most of the moons were discovered in the late 1970s and later as a result of several explorations by automated spacecraft, including NASA’s Voyager in 1979 and Galileo in 1995. Not only is Jupiter the largest planet in the solar system , it is also the most massive at more than 300 times the mass of Earth. Its size plays a role in the number of moons orbiting Jupiter because there is a large area of gravitational stability around it to support many moons. Jupiter also has the strongest magnetic field of any planet, so anything passing near it, such as an asteroid, is either destroyed by gravitational tides or captured into its orbit. Earth only has one moon because it lacks the strong gravitational field and mass necessary to hold another satellite in orbit. The moons of Jupiter have orbital periods ranging from seven hours to almost three Earth years. Some of the orbits are nearly circular, while the moons farthest from Jupiter have more irregular orbits.

The outer 33 moons orbit in the opposite direction in which Jupiter spins, which is unusual and indicates the moons were asteroids that were sucked into Jupiter’s orbit after the initial system was formed. A translation of the key passages of Galileo Galilei's journal detailing his discovery of four moons orbiting Jupiter. Galileo made these sketches after discovering four moons orbiting Jupiter in January 1610. The moons, later named Io, Europa, Callisto and Ganymede, were the first discovered beyond Earth. Credit: NASA Galilean moons In January 1610, Italian astronomer Galileo Galilei discovered four of Jupiter’s moons — now called Io, Europa, Ganymede, and Callisto. He originally referred to the individual moons numerically as I, II, III, and IV. The numerical system for naming the moons lasted for a few centuries until scientists determined that simply using numbers as a naming device would be confusing and impractical as more moons were discovered. Galileo’s discovery was pivotal point in the history of astronomy as his observation revealed that not all celestial bodies revolved around the Earth. Until that time, Earth was thought to be the center of the universe.

Eight satellites — the four Galilean and four smaller moons — are closer to the planet and provide the dust that make up Jupiter's rings. The closest of the Galilean moons to Jupiter is Io, the first moon to be discovered by Galileo . This satellite’s distinctive feature is its volcanoes, making it the only celestial body in the solar system besides Earth to have volcanic activity. This moon also has sulfur dioxide snowfields, leading to its characterization as a moon of fire and ice. Io has an iron or iron sulfide core and a brown silicate outer layer, which gives it a splotchy orange, yellow, black, red, and white appearance. Moving outward from Jupiter is Europa . While slightly smaller than Earth’s moon, it is still one of the largest bodies in the solar system but the smallest of the Galilean satellites. Cracks and streaks crisscross the entire icy surface, which is marked with very few craters. Europa has a high degree of reflectivity, making it among the brightest moons in the solar system. At 20 to 180 million years old, the surface is fairly young. It is possible that an extensive ocean beneath the surface harbors life.

Ganymede is the third Galilean moon from Jupiter and the largest of the four. This low-density moon is about the size of Mercury but has about half the mass. Its outstanding characteristic is that it is the only moon to have its own magnetic field. The satellite’s iron core is topped off by a thick crust that is mostly ice. Forty percent of the surface of Ganymede is covered by highly cratered dark regions, and the remaining sixty percent is covered by a light grooved terrain, which forms intricate patterns across Ganymede. Callisto , the fourth and farthest of the Galilean moons from Jupiter, is the most heavily cratered object in the solar system. The moon’s landscape has essentially remained unchanged since its formation, which has garnered much interest among scientists. It is about the size of Mercury but very low in density. It is also experiences the least impact of Jupiter’s magnetic field as its orbit is the farthest from the planet and beyond Jupiter’s primary radiation belt.

— Kim Ann Zimmermann, SPACE.com Contributor Related: Moons of Jupiter Moons of Jupiter The Moons of Jupiter by Dominic Ford The charts below show the changing positions of Jupiter's four largest Moons, which are easily visible through a pair of modest binoculars or a small telescope as a series of pinpoints of light around Jupiter. Indeed, Galileo first discovered them in 1610 them using a telescope which only offered 20× magnification. The east-west position of each moon relative to Jupiter is shown by a colored line. The planet's disk is represented by the gray strip that runs down the center of each column, which the moons past in front of when travelling from east to west (left to right), and behind when travelling from west to east (right to left). Date Planet Jupiter: Facts About Its Size, Moons and Red Spot Planet Jupiter: Facts About Its Size, Moons and Red Spot By Charles Q. Choi, Space.com Contributor | November 14, 2014 12:59am ET MORE This photo of Jupiter was taken on Sept. 20, 2010 when Jupiter made its closest approach to Earth since 1963.

(Uranus [insert] was visible through telescopes near Jupiter.) Credit: Jimmy Eubanks Jupiter is the largest planet in the solar system. Fittingly, it was named after the king of the gods in Roman mythology. In a similar manner, the ancient Greeks named the planet after Zeus, the king of the Greek pantheon. Jupiter helped revolutionize the way we saw the universe and ourselves in 1610, when Galileo discovered Jupiter's four large moons — Io, Europa, Ganymede and Callisto, now known as the Galilean moons. This was the first time celestial bodies were seen circling an object other than Earth, major support of the Copernican view that Earth was not the center of the universe. Physical characteristics Jupiter is the most massive planet in our solar system , more than twice as massive as all the other planets combined, and had it been about 80 times more massive, it would have actually become a star instead of a planet. Its atmosphere resembles that of the sun, made up mostly of hydrogen and helium, and with four large moons and many smaller moons in orbit around it, Jupiter by itself forms a kind of miniature solar system.

All told, the immense volume of Jupiter could hold more than 1,300 Earths. The colorful bands of Jupiter are arranged in dark belts and light zones created by strong east-west winds in the planet's upper atmosphere traveling more than 400 mph (640 kph). The white clouds in the zones are made of crystals of frozen ammonia, while darker clouds of other chemicals are found in the belts. At the deepest visible levels are blue clouds. Far from being static, the stripes of clouds change over time . Inside the atmosphere, diamond rain may fill the skies. The most extraordinary feature on Jupiter is undoubtedly the Great Red Spot , a giant hurricane-like storm seen for more than 300 years. At its widest, the Great Red Spot is three times the diameter of the Earth, and its edge spins counterclockwise around its center at a speed of about 225 mph (360 kph). The color of the storm, which usually varies from brick red to slightly brown, may come from small amounts of sulfur and phosphorus in the ammonia crystals in Jupiter's clouds. The spot grows and shrinks over time, and every now and again, seems to fade entirely.

Jupiter's gargantuan magnetic field is the strongest of all the planets in the solar system at nearly 20,000 times the strength of Earth's. It traps electrically charged particles in an intense belt of electrons and other electrically charged particles that regularly blasts the planet's moons and rings with a level of radiation more than 1,000 times the lethal level for a human, damaging even heavily shielded spacecraft such as NASA's Galileo probe. The magnetosphere of Jupiter, which is composed of these fields and particles, swells out some 600,000 to 2 million miles (1 million to 3 million km) toward the sun and tapers to a tail extending more than 600 million miles (1 billion km) behind Jupiter. Jupiter spins faster than any other planet, taking a little under 10 hours to complete a turn on its axis, compared with 24 hours for Earth. This rapid spin makes Jupiter bulge at the equator and flatten at the poles, making the planet about 7 percent wider at the equator than at the poles. Jupiter broadcasts radio waves strong enough to detect on Earth.

These come in two forms — strong bursts that occur when Io, the closest of Jupiter's large moons, passes through certain regions of Jupiter's magnetic field, and continuous radiation from Jupiter's surface and high-energy particles in its radiation belts. These radio waves could help scientists to probe the oceans on its moons. Composition & structure Atmospheric composition (by volume): 89.8 percent molecular hydrogen, 10.2 percent helium, minor amounts of methane, ammonia, hydrogen deuteride, ethane, water, ammonia ice aerosols, water ice aerosols, ammonia hydrosulfide aerosols Magnetic field: Nearly 20,000 times stronger than Earth's Chemical composition: Jupiter has a dense core of uncertain composition , surrounded by a helium-rich layer of fluid metallic hydrogen, wrapped up in an atmosphere primarily made of molecular hydrogen. Internal structure: A core less than 10 times Earth's mass surrounded by a layer of fluid metallic hydrogen extending out to 80 to 90 percent of the diameter of the planet, enclosed in an atmosphere mostly made of gaseous and liquid hydrogen.

Orbit & rotation Average distance from the sun : 483,682,810 miles (778,412,020 km). By comparison: 5.203 times that of Earth Perihelion (closest approach to the sun): 460,276,100 miles (740,742,600 km). By comparison: 5.036 times that of Earth Aphelion (farthest distance from the sun): 507,089,500 miles (816,081,400 km). By comparison: 5.366 times that of Earth (Source: NASA .) Jupiter's moons Jupiter has at least 63 moons , which are often named after the Roman god's many lovers. The four largest moons of Jupiter, now called Io, Europa, Ganymede, and Callisto, were discovered by Galileo Galilei himself, and are appropriately known today as the Galilean satellites. Ganymede is the largest moon in our solar system, larger even than Mercury and Pluto.

It is also the only moon known to have its own magnetic field. The moon has at least one thick ocean between layers of ice, although it may contain several layers of both materials. Io is the most volcanically active body in our solar system. The sulfur its volcanoes spew out gives Io a blotted yellow-orange appearance that is often compared to a pepperoni pizza. As Io orbits Jupiter, the planet's immense gravity causes 'tides' in Io's solid surface that rise 300 feet (100 meters) high, generating enough heat for volcanic activity. The frozen crust of Europa is made up mostly of water ice, and it may hide a liquid ocean holding twice as much water as Earth does. Icy oceans may also exist beneath the crusts of Callisto and Ganymede. Some of this liquid spouts from the surface in newly spotted sporadic plumes at the southern pole. Its potential to host life caused NASA to request funding for a mission to explore Europa . Callisto has the lowest reflectivity, or albedo, of the four Galilean moons. This suggests that its surface may be composed of dark, colorless rock.

Jupiter's rings Jupiter's three rings came as a surprise when NASA's Voyager 1 spacecraft discovered them around the planet's equator in 1979. Each are much fainter than Saturn's rings. The main ring is flattened. It is about 20 miles (30 km) thick and more than 4,000 miles (6,400 km) wide. The inner cloud-like ring, called the halo, is roughly 12,000 miles (20,000 km) thick. The halo extends halfway from the main ring down to the planet's cloud tops and expands by interaction with Jupiter's magnetic field. Both the main ring and halo are composed of small, dark particles. The third ring, known as the gossamer ring because of its transparency, is actually three rings of microscopic debris from three of Jupiter's moons, Amalthea, Thebe and Adrastea.

It is probably made up of dust particles less than 10 microns in diameter, about the same size of the particles found in cigarette smoke, and extends to an outer edge of about 80,000 miles (129,000 km) from the center of the planet and inward to about 18,600 miles (30,000 km). Ripples in the rings of both Jupiter and Saturn may be signs of impacts from comets and asteroids. Research & exploration Seven missions have flown by Jupiter — Pioneer 10 , Pioneer 11 , Voyager 1 , Voyager 2 , Ulysses, Cassini and New Horizons — while another, NASA's Galileo, actually orbited the planet. Pioneer 10 revealed how dangerous Jupiter's radiation belt is, while Pioneer 11 provided data on the Great Red Spot and close-up pictures of its polar region. Voyager 1 and 2 helped astronomers create the first detailed maps of the Galilean satellites, discovered Jupiter's rings, revealed sulfur volcanoes on Io, and saw lightning in Jupiter's clouds.

Ulysses discovered the solar wind has a much greater impact on Jupiter's magnetosphere than before suggested. New Horizons took close-up pictures of Jupiter and its largest moons. In 1995, Galileo sent a probe plunging towards Jupiter, making the first direct measurements of its atmosphere and measuring the amount of water and other chemicals there. When Galileo ran low on fuel, the craft was intentionally crashed into Jupiter's atmosphere to avoid any risk of it slamming into and contaminating Europa, which might have an ocean below its surface capable of supporting life. Another spacecraft, named Juno , is heading toward Jupiter and will reach the planet in 2016. It will study Jupiter from a polar orbit to figure out how it and the rest of the solar system formed, which could shed light on how alien planetary systems might have developed. Jupiter's gravitational impact on the solar system As the most massive body in the solar system after the sun, the pull of Jupiter's gravity has helped shape the fate of our system. It may have violently hurled Neptune and Uranus outward , according to calculations published in the journal Nature.

Jupiter, along with Saturn, may have slung a barrage of debris toward the inner planets early in the system's history, according to an article in Science magazine. It may even nowadays help keep asteroids from bombarding Earth, and recent events certainly have shown that it can absorb potentially deadly impacts . Currently, Jupiter's gravitational field influences numerous asteroids that have clustered into the regions preceding and following Jupiter in its orbit around the sun. These are known as the Trojan asteroids, after three large asteroids there, Agamemnon, Achilles and Hector, names drawn from the Iliad, Homer's epic about the Trojan War. Possibility of life on Jupiter If one were to dive into Jupiter's atmosphere , one would discover it to grow warmer with depth, reaching room temperature, or 70 degrees F (21 degrees C), at an altitude where the atmospheric pressure is about 10 times as great as it is on Earth. Scientists have conjectured that if Jupiter has any form of life, it might dwell at this level, and would have to be airborne. However, researchers have found no evidence for life on Jupiter.

Additional reporting by Nola Taylor Redd, Space.com Contributor Explore the solar system The Galilean Moons of Jupiter The Galilean Moons of Jupiter Summary: Jupiter has more than 60 known moons, but understanding the geology of its four largest will hopefully lead to some groundbreaking discoveries. Sections: The Galilean Moons Each of the Jovian planets has a number of moons, although Jupiter has the most with more than 60 catalogued to date. Jupiter's 4 largest moons exhibit some of the most interesting geology in the solar system. They were discovered by Galileo Galilei and are known as the Galilean moons. Jupiter's largest moon, Ganymede, is larger than Mercury while the other three are larger than Pluto. Outward from Jupiter: Io: Volcanoes and All Jupiter's Moon Io The Voyager spacecraft took the first close-up images of Io more than 300 years after the moon's discovery. The images showed a surface with no signs of craters from past impacts. What we saw instead was a surface almost entirely covered with large volcanoes. Cameras on Voyager actually captured volcanic eruptions in progress.

The frequency of these sulfuric eruptions has filled in almost all of the impact craters and left Io with one of the youngest looking surfaces in the solar system. Io Close-Up (click to enlarge) Close-up photos of eruptions in progress show powerfully hot lava glowing orange and red. Photos taken on the night side of Io show not only the hot volcanic vents, but also a thin sulfur dioxide atmosphere produced by constant outgassing. Io's unusual red and orange colors come primarily from sulfur, which condenses on the surface after being outgassed by the volcanoes. Although there is no direct evidence of tectonic activity on Io, scientists feel confident it exists since the processes that fuel volcanism also fuel tectonics. The volcanic eruptions are so frequent and cover the surface so thoroughly that any clear evidence of tectonic activity is likely to be buried. Europa: What Lies Beneath? Europa Europa's surface and crust are made almost entirely of water ice, and its bizarre, fractured appearance is proof enough that tidal heating has acted there. The icy surface is nearly devoid of impact craters and may be only a few million years old.

Europa's Interior Observations made by the Galileo spacecraft show that Europa has a metallic core and a rocky mantle. Surrounding the rocky interior appears to be an icy layer 100 kilometers thick, the top few kilometers of which seem to be frozen solid. The stretching and squeezing of tidal friction should provide enough heat to melt some of this into liquid water beneath a thin ice shell. If it does, then Europa may have an ocean with more than twice as much liquid water as all of Earth's oceans combined. Analyzing Europa's Cracked Surface Close-up photos of the surface of Europa support the idea of a liquid ocean beneath the surface. These photos, taken by the Galileo spacecraft, show what appears to be icebergs stuck in a layer of ice. Other evidence comes from double-ridged cracks on the surface. Tidal flexing that allows water to well up and build ridges may create these cracks. Ganymede: Largest Moon in the Solar System Two Regions of Ganymede's Surface (click to enlarge) The surface of Ganymede shares many similarities with Europa. Ganymede's surface is also made of water ice, but unlike Europa's surface, it shows signs of varying age.

The darker regions are heavily cratered, suggesting they are billions of years old. The lighter regions show no signs of craters and it is thought that eruptions of water covered the surface before freezing over. These areas are geologically younger than the darker regions. If liquid water occasionally makes its way to the surface to fill in craters, could that suggest a liquid ocean similar to the one that might exist on Europa? Callisto: The Outermost Galilean Moon Callisto Callisto is the stereotypical outer solar system satellite. It is one of the largest and most heavily cratered satellites in the solar system. The surface is very icy and dates back four billion years. Beneath the icy crust is possibly a salty ocean supported by a deeper rocky interior. Callisto's Surface Callisto doesn't have any large mountains, show evidence of volcanic or tectonic activity or have any appreciable level of internal heat. Nonetheless, observations of Callisto's magnetic field may cause scientists to add the large moon to the list of possible worlds with subsurface salty oceans. NOVA | Galileo: Discovering Jupiter'... Log in to Teachers' Domain to download, share, rate, save, and match to state standards.

Source: NOVA: "Galileo's Battle for the Heavens" This resource was adapted from NOVA: "Galileo's Battle for the Heavens." Background In 1610, when Galileo first pointed his newly built telescope toward the sky, no one had ever had the capability to see distant objects in any detail, and he had no idea what to expect. Focusing on the Moon first, he discovered that its surface was not smooth as people had previously thought, but covered with craters and mountains not unlike those on Earth. Next, Galileo turned his attention to another bright object in the night sky — the planet Jupiter. Today, we know quite a bit about Jupiter. With a diameter about 11 times greater than Earth's, Jupiter is the largest planet around our Sun. It is so enormous that its mass is more than twice the combined mass of all the other planets in our solar system. Composed mostly of gaseous hydrogen and helium, Jupiter is a gas giant. It has a trademark banded look that results from its turbulent atmosphere, and its Great Red Spot is a giant storm that has been raging for centuries.

Jupiter also has a faint ring system and, as of May 2005, 63 known moons surround it. In a very short time, Galileo discovered Jupiter's four largest moons — Io, Europa, Ganymede, and Callisto, now called the Galilean moons. Each moon has unique characteristics that make it stand out in the solar system. For example, Io is the most volcanically active object in the solar system. Europa may have an ocean of liquid water beneath its icy surface and may be the best candidate for another habitable world. Callisto is one of the most heavily cratered bodies in the solar system, and Ganymede, which is even larger than the planet Mercury, is the largest moon. The four Galilean moons were the first celestial bodies to be discovered with a telescope. Although Galileo at first thought they were fixed stars, his continued observations showed that their positions changed on a nightly basis. Galileo soon recognized that these new objects were satellites of Jupiter. The discovery that celestial objects could revolve around something other than Earth provided strong evidence in support of the heliocentric theory. To learn more about early telescopes, check out The Beginnings of the Telescope and Galileo's Telescope .

To learn more about how Galileo's observations changed astronomy, check out Galileo: Sun-Centered System . To learn more about another of Galileo's astronomical discoveries, check out Galileo: Sunspots . Questions for Discussion How did Galileo surmise that Jupiter had moons that revolved around it? How did the discovery of Jupiter's moons support the heliocentric theory? What was so shocking about this discovery in Galileo's time? Why did he rush to publish his discovery? Explain the importance of Galileo's work on the telescope and its effect on the world at that time and today. Discuss the role of technology in science. The Galileo Project | Science | Satellites of Jupiter Jupiter's moons Satellites of Jupiter Jupiter has a large number of satellites. Of these, four are comparable to the Earth's Moon in size; the rest are orders of magnitude smaller. When Jupiter is at opposition and closest to the Earth, the stellar magnitude of its four large moons is between 5 and 6. [1] This means that, were it not for the shielding brightness of Jupiter, these bodies would be visible with the naked eye.

The aperture of the telescope used by Galileo in 1610 and its magnification thus brought these four "Galilean" satellites within his grasp. But first Galileo had to make adjustments to the instruments. When viewing bodies that are very bright and very small, the optical defects of the telescope can be crippling. By trial and error Galileo learned to stop down the aperture of his instrument until he could begin to make useful observations. At the end of 1609, as he was finishing his series of observations of the Moon , Jupiter was at opposition and the brightest object in the evening sky (not counting the Moon). When he had made the new adjustment to his instrument, he turned his attention to Jupiter. On 7 January 1610 he observed the planet and saw what he thought were three fixed stars near it, strung out on a line through the planet. This formation caught his attention, and he returned to it the following evening. Galileo's expectation was that Jupiter, which was then in its retrograde loop, [2] would have moved from east to west and had left the three little stars behind. Instead, he saw all three stars to the west of Jupiter.

It appeared as though Jupiter had not moved to the west but rather to the east. This was an anomaly, and Galileo returned to this formation again and again. Over the next week he found out several things. First, the little stars never left Jupiter; they appeared to be carried along with the planet. Second, as they were carried along, they changed their position with respect to each other and Jupiter. Third, there were not three but four of these little stars. By the 15th of January he had figured it out: these were not fixed stars but rather planetary bodies that revolved around Jupiter. Jupiter had four moons. His book, Sidereus Nuncius, in which his discovery was described, came off the press in Venice in the middle of March 1610 and made Galileo famous. Galileo's observations of Jupiter's moons (from manuscripts) [click for larger image] The moons of Jupiter had a major impact on cosmology. In 1610 the traditional Aristotelian cosmology had come under attacks from Copernican astronomers. Aristotelians had a number of arguments against the Copernican System, one of which was now made obsolete.

In traditional cosmology, there was only one center of motion, the center of the universe which was the place of the Earth. The motions of all heavenly bodies centered on the Earth. But according to the Copernican theory, the Earth went around the Sun while the Moon went around the Earth. There were thus two centers of motion, which seemed an absurdity. Moreover, if the Earth was a planet, like Mercury, Venus, Mars, Jupiter, and Saturn, why was it the only planet to have a Moon? Galileo's discovery answered this question. The Earth was, in fact, not the only planet to have a moon, Jupiter had four. And no matter what cosmological system one believed in, there were now at least two centers of motion in the universe, the Earth or Sun and Jupiter. Thus, although the satellites (the term was first used by Johannes Kepler ) of Jupiter were by no means proof of the truth of the Copernican system, they certainly added ammunition on that side of the argument. In the purely astronomical realm, the satellites of Jupiter posed a new problem for astronomers. It had taken centuries in Antiquity to arrive at adequate geometrical modes for the motions of the known planets.

Now there was a new system of planetary bodies in miniature, and astronomers had to develop models that could predict their motions. There was a great incentive to come up with good mathematical models, for the satellites offered some hope for the solution of the problem of longitude at sea . It took almost two centuries, however, before the models and tables based on them reached satisfactory accuracy. The naming of the satellites provides an interesting example of how such matters were handled before the foundation of the International Astronomical Union in the twentieth century. As their discoverer, Galileo claimed the right to name the satellites. He wanted to name them after his patrons and asked whether they would prefer "Cosmic Stars" (after Cosimo II ) or "Medicean Stars." They opted for the latter, and through much of the seventeenth century they were known by that name. In his notebooks, Galileo referred to them individually by number, starting with the satellite closest to Jupiter, but he never had occasion to refer to them in this way in print. In Provence, Nicholas Claude Fabri de Peiresc tried to differentiate between the Medicean Stars by assigning them the names of individual members of the family, but this system was not published and thus was never used by others.

In his Mundus Iovialis ("Jovian World") of 1614, Simon Marius went into the naming problem in some depth. First, he himself used the numerical system beginning with the satellite closest to Jupiter. Second, he thought that he might call them after his patron, the Duke of Brandenburg -- a suggestion followed by no one. Third, he suggested naming the farthest satellite the Saturn of Jupiter, the next one the Jupiter of Jupiter, the third one the Venus of Jupiter, and the one nearest the planet the Mercury of Jupiter. This cumbersome system never caught on. Finally, Marius related a suggestion by Kepler : Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River, Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell . . . .

I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto . . . . This fancy, and the particular names given, were suggested to me by Kepler, Imperial Astronomer, when we met at Ratisbon fair in October 1613. So if, as a jest, and in memory of our friendship then begun, I hail him as joint father of these four stars, again I shall not be doing wrong. [3] None of these suggestion caught on because with Jupiter's satellites , there was no confusion in the numbering system. Following Galileo and Marius, astronomers simply referred to them by number. With the satellites of Saturn, however, a problem developed. In 1655 Huygens discovered the first and largest; then in 1671-72 Giandomenico Cassini discovered two more, and in 1684 yet another two. These five satellites were numbered like their Galilean counterparts. But when in 1789 William Herschel discovered two additional satellites internal to the first, confusion followed.

Did one now renumber them all (thus causing confusion for those who consulted older works), refer to the two new ones as nos. 6 and 7 (thus making the order of the satellites confusing), or refer to them by order of discovery (equally confusing as to order)? Herschel's son, John Frederick William, suggested in 1847 that Saturn's satellites be given individual names of mythological figures associated with Saturn after the suggestion made by Marius for Jupiter's satellites. When, the following year, William Lassel and George Bond independently discovered an eighth satellite of Saturn, they agreed to adopt the naming system proposed by Herschel, in which Saturn's satellites were named after his brothers and sisters, the Titans. This system and the now revived suggestion by Kepler and Marius for Jupiter quickly became the convention for naming the satellites of the superior planets. Modern Images of the Galilean Satellites Io and Europa

Question: Who discovered the four largest moons of Jupiter around 1610?
Answer: GALILEO